diff --git a/library/x509write.c b/library/x509write.c
new file mode 100644
index 0000000..cd3c739
--- /dev/null
+++ b/library/x509write.c
@@ -0,0 +1,186 @@
+/*
+ *  X.509 internal, common functions for writing
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+#include "common.h"
+#if defined(MBEDTLS_X509_CSR_WRITE_C) || defined(MBEDTLS_X509_CRT_WRITE_C)
+
+#include "mbedtls/x509_crt.h"
+#include "mbedtls/asn1write.h"
+#include "mbedtls/error.h"
+#include "mbedtls/oid.h"
+#include "mbedtls/platform.h"
+#include "mbedtls/platform_util.h"
+#include "mbedtls/md.h"
+
+#include <string.h>
+#include <stdint.h>
+
+#if defined(MBEDTLS_PEM_WRITE_C)
+#include "mbedtls/pem.h"
+#endif /* MBEDTLS_PEM_WRITE_C */
+
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
+#include "psa/crypto.h"
+#include "mbedtls/psa_util.h"
+#include "md_psa.h"
+#endif /* MBEDTLS_USE_PSA_CRYPTO */
+
+#define CHECK_OVERFLOW_ADD(a, b) \
+    do                         \
+    {                           \
+        if (a > SIZE_MAX - (b)) \
+        { \
+            return MBEDTLS_ERR_X509_BAD_INPUT_DATA; \
+        }                            \
+        a += b; \
+    } while (0)
+
+int mbedtls_x509_write_set_san_common(mbedtls_asn1_named_data **extensions,
+                                      const mbedtls_x509_san_list *san_list)
+{
+    int ret = 0;
+    const mbedtls_x509_san_list *cur;
+    unsigned char *buf;
+    unsigned char *p;
+    size_t len;
+    size_t buflen = 0;
+
+    /* Determine the maximum size of the SubjectAltName list */
+    for (cur = san_list; cur != NULL; cur = cur->next) {
+        /* Calculate size of the required buffer */
+        switch (cur->node.type) {
+            case MBEDTLS_X509_SAN_DNS_NAME:
+            case MBEDTLS_X509_SAN_UNIFORM_RESOURCE_IDENTIFIER:
+            case MBEDTLS_X509_SAN_IP_ADDRESS:
+            case MBEDTLS_X509_SAN_RFC822_NAME:
+                /* length of value for each name entry,
+                 * maximum 4 bytes for the length field,
+                 * 1 byte for the tag/type.
+                 */
+                CHECK_OVERFLOW_ADD(buflen, cur->node.san.unstructured_name.len);
+                CHECK_OVERFLOW_ADD(buflen, 4 + 1);
+                break;
+            case MBEDTLS_X509_SAN_DIRECTORY_NAME:
+            {
+                const mbedtls_asn1_named_data *chunk = &cur->node.san.directory_name;
+                while (chunk != NULL) {
+                    // Max 4 bytes for length, +1 for tag,
+                    // additional 4 max for length, +1 for tag.
+                    // See x509_write_name for more information.
+                    CHECK_OVERFLOW_ADD(buflen, 4 + 1 + 4 + 1);
+                    CHECK_OVERFLOW_ADD(buflen, chunk->oid.len);
+                    CHECK_OVERFLOW_ADD(buflen, chunk->val.len);
+                    chunk = chunk->next;
+                }
+                CHECK_OVERFLOW_ADD(buflen, 4 + 1);
+                break;
+            }
+            default:
+                /* Not supported - return. */
+                return MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE;
+        }
+    }
+
+    /* Add the extra length field and tag */
+    CHECK_OVERFLOW_ADD(buflen, 4 + 1);
+
+    /* Allocate buffer */
+    buf = mbedtls_calloc(1, buflen);
+    if (buf == NULL) {
+        return MBEDTLS_ERR_ASN1_ALLOC_FAILED;
+    }
+    p = buf + buflen;
+
+    /* Write ASN.1-based structure */
+    cur = san_list;
+    len = 0;
+    while (cur != NULL) {
+        size_t single_san_len = 0;
+        switch (cur->node.type) {
+            case MBEDTLS_X509_SAN_DNS_NAME:
+            case MBEDTLS_X509_SAN_RFC822_NAME:
+            case MBEDTLS_X509_SAN_UNIFORM_RESOURCE_IDENTIFIER:
+            case MBEDTLS_X509_SAN_IP_ADDRESS:
+            {
+                const unsigned char *unstructured_name =
+                    (const unsigned char *) cur->node.san.unstructured_name.p;
+                size_t unstructured_name_len = cur->node.san.unstructured_name.len;
+
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_asn1_write_raw_buffer(
+                                                 &p, buf,
+                                                 unstructured_name, unstructured_name_len));
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len, mbedtls_asn1_write_len(
+                                                 &p, buf, unstructured_name_len));
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_asn1_write_tag(
+                                                 &p, buf,
+                                                 MBEDTLS_ASN1_CONTEXT_SPECIFIC | cur->node.type));
+            }
+            break;
+            case MBEDTLS_X509_SAN_DIRECTORY_NAME:
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_x509_write_names(&p, buf,
+                                                                      (mbedtls_asn1_named_data *) &
+                                                                      cur->node
+                                                                      .san.directory_name));
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_asn1_write_len(&p, buf, single_san_len));
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_asn1_write_tag(&p, buf,
+                                                                    MBEDTLS_ASN1_CONTEXT_SPECIFIC |
+                                                                    MBEDTLS_ASN1_CONSTRUCTED |
+                                                                    MBEDTLS_X509_SAN_DIRECTORY_NAME));
+                break;
+            default:
+                /* Error out on an unsupported SAN */
+                ret = MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE;
+                goto cleanup;
+        }
+        cur = cur->next;
+        /* check for overflow */
+        if (len > SIZE_MAX - single_san_len) {
+            ret = MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+            goto cleanup;
+        }
+        len += single_san_len;
+    }
+
+    MBEDTLS_ASN1_CHK_CLEANUP_ADD(len, mbedtls_asn1_write_len(&p, buf, len));
+    MBEDTLS_ASN1_CHK_CLEANUP_ADD(len,
+                                 mbedtls_asn1_write_tag(&p, buf,
+                                                        MBEDTLS_ASN1_CONSTRUCTED |
+                                                        MBEDTLS_ASN1_SEQUENCE));
+
+    ret = mbedtls_x509_set_extension(extensions,
+                                     MBEDTLS_OID_SUBJECT_ALT_NAME,
+                                     MBEDTLS_OID_SIZE(MBEDTLS_OID_SUBJECT_ALT_NAME),
+                                     0,
+                                     buf + buflen - len, len);
+
+    /* If we exceeded the allocated buffer it means that maximum size of the SubjectAltName list
+     * was incorrectly calculated and memory is corrupted. */
+    if (p < buf) {
+        ret = MBEDTLS_ERR_ASN1_LENGTH_MISMATCH;
+    }
+cleanup:
+    mbedtls_free(buf);
+    return ret;
+}
+
+#endif /* MBEDTLS_X509_CSR_WRITE_C || MBEDTLS_X509_CRT_WRITE_C */